The present invention relates to a method and apparatus for calibrating temperature measurements that are taken with at least one first radiation detector for measuring heat radiation emitted by at least one substrate.
Methods and apparatus of this type are known, for example, in connection with the production of semiconductor substrates in a reaction chamber. Preferred in such chambers are methods that are independent of emissivity and in which heat radiation coming from a first substrate is compared to the actual temperature of the substrate largely independent of the current emissivity. Such a method compensates differences in emissivity between different substrates. Used for achieving a measurement method that is independent of emissivity are, for example, the so-called xe2x80x9cripple techniquexe2x80x9d, which is described, for example in U.S. Pat. No. 5,490,728 and in DE 197 54 386 A (not previously published) from the same applicant, and the cavity principle, in which, for example, a mirrored chamber is closed on one side by an article to be measured, whereby an approximation of a cavity radiator with emissivity 1 is achieved. Another method that uses the cavity principle is described, for example, in DE 197 37 802 (not previously published) from the same applicant, which is incorporated into this application in order to avoid repetition. In the known methods wafers are used and a thermoelement (TCxe2x80x94ThermoCouple) is glued to the tops or bottoms thereof. Experience has demonstrated that the measurement deviation from TC to TC is very small as long as the TCs were produced from pairs of wires in the same series. The deviation then ranges from about 1 to 2xc2x0 K. Based on this slight deviation, the temperature measurements can be adjusted to be independent of emissivity since in this case the issue in particular is slight deviation between the TCs. The uncertainty in the measurement of the absolute temperature is substantially greater, however, whereby the measurement uncertainty is at best in the region of 2-3 K and is more than 10 K-20 K in less favorable cases.
These measurement uncertainties are the result of different factors. Among these factors is that the thermoelectromotive force of the thermopair depends not only on the temperature but also on the alloy, but it is subject to certain deviation determined by production factors. Furthermore, provided between the TC and its amplifier are a plurality of electrical connections that also constitute the thermopair, so that unsymmetrical transitions produce additional thermoelectromotive force. In addition, the adhesive sites for the TCs have a different degree of absorption than the wafer surface surrounding the adhesive sites. The equilibrium temperature of the TCs is thus not determined solely by means of heat conducted between wafer and TC, which would be ideal, but rather the temperature of the TC is also influenced by the heat radiated from the lamp, which is why the temperature of the TCs frequently does not precisely equal the temperature of the substrate.
The measurement uncertainties thus arise primarily from erroneous sources that affect all TCs in the same manner as measurement errors. TCs are therefore measurement recorders that have great absolute measurement uncertainty, although they have a low deviation.
For achieving improved measurement accuracy, the TCs were calibrated in a TC calibrator, i.e., in an oven with very homogenous temperature distribution in the interior in terms of absolute temperature measurement. A plurality of uncalibrated TCs were placed in the oven together with a reference TC, whereby the reference TC itself was calibrated by a separate calibration service using a transfer pyrometer with respect to a primary reference. These multiple calibrations require different apparatus, they are very expensive, and, due to the many steps involved, there are numerous opportunities for errors to be introduced into the calibration process, which could again result in measurement uncertainties at the end of the process.
Furthermore, known from U.S. Pat. No. 5,265,957 is an apparatus and a method for calibrating a temperature sensor in which a wafer is provided with a plurality of calibration islands made of a reference material with a melting point in the range of 150xc2x0 C. to 550xc2x0 C. While such a wafer is being heated, the effective reflectivity of the wafer is measured by the temperature sensor and a first step change in an output signal of the temperature sensor is equated to a wafer temperature that equals the melting point of the reference material. Then temperature sensor calibration parameters are calculated. This principle is illustrated in FIG. 6. The signal I of a temperature sensor (pyrometer) is recorded as a function of time t while the wafer is heated. For the reflectivity of the wafer to change when the reference material reaches phase transition, the reference material must be arranged close to the surface so that it is in the range of the penetration depth of the measurement wavelength. The aforementioned step change in the pyrometer signal occurs at the phase transition of the reference material, as shown in FIG. 6. The method shown in U.S. Pat. No. 5,265,957 has substantial disadvantages. For instance, no unique pyrometer value can be allocated to melting point Tm1 due to the step change in the pyrometer signal, which results in a systematic measurement error xcex94I for the calibration method.
The object of the invention is therefore to provide a method and an apparatus of the type cited in the foregoing in which temperature measurements can be calibrated with great accuracy in a simple and cost-effective manner.
This object is inventively achieved using a method of the type described in the foregoing that has the following process steps: heating a reference substrate carrying at least one reference material with a known melting point to the melting point or over the melting point; measuring the thermal radiation of the reference substrate during the heating and/or during a cooling period following the heating; comparing a measurement plateau during the measurement process to the known melting point.
Heating the reference material located on the reference substrate raises the temperature of the reference substrate and the reference material until it reaches the melting point of the reference material. Once the melting point has been achieved, the temperature does not increase further until all of the reference material has converted to the liquid phase, that is, until the latent heat has been conveyed to the reference material. During cooling, this process reverses in a known manner. Since the melting point of the reference material is known precisely, it is possible to compare a measurement plateau measured during the heating and/or during a cooling period following the heating to the known melting point, thus achieving simple calibration of an absolute temperature measurement.
Advantageously the measurement plateau is determined during the heating and/or cooling of the reference substrate. However, the measurement plateau is preferably determined during the cooling period since the reference material in its melted state prior to complete solidification has particularly good contact to the reference substrate in terms of heat conductivity.
The method in accordance with the invention has substantial advantages compared to the method in U.S. Pat. No. 5,265,957 described in the foregoing. Since the method does not depend on changes in the emissivity of the reference substrate, it is possible to surround the reference material with a thick protective coating or to arrange the reference material in the interior of the reference substrate. It is not necessary to arrange it near the surface. The advantage of this is that the reference material cannot contaminate the processing chamber. This is the basic prerequisite for broad use of the reference substrate in semiconductor technology.
An additional advantage results from the occurrence of a measurement plateau at the known melting point, as illustrated in FIG. 7. In contrast to the method described in U.S. Pat. No. 5,265,957, in the method in accordance with the invention, due to the plateau, it is possible to clearly allocate a radiation detector signal to a melting point. The system error xcex94I mentioned in the foregoing is minimized in the method in accordance with the invention.
Formation of a plateau furthermore has the advantage that during the time tp, thermal equilibrium occurs between the reference material and the reference substrate, thereby minimizing possible temperature differences. It is furthermore advantageous that optical properties of the surface of the reference substrate do not change during the method in accordance with the invention. For instance, if the reference substrate (with the exception of a reference material located in the interior) is selected from the same material as the substrate to be processed later (e.g. Si), the reference substrate has the same emissivity as this substrate. This means that the radiation detector can be calibrated directly while substrates are being processed, even without compensating for emissivity. It is only when the substrates have a different (spectral) emissivity, e.g. due to different surface quality, that a correction has to be undertaken or the emissivity has to be compensated. If it is possible to process the substrate without emissivity compensation, this has the further advantage that only one radiation detector is required for the temperature measurement. Such a temperature measurement method with only one radiation detector is not possible with the method described in U.S. Pat. No. 5,265,957 because in this method the emissivity of the surface of the reference substrate changes when the phase changes.
In order for the plateau to be easily detectable in terms of metrology, the mass of the reference material should be at least 1% of the total mass of the reference substrate. This low mass ratio can be selected because the specific melting heat is a multiple of the specific heat capacity. In order for the optical properties of the surface of the reference substrate not to change, the protective coating of the reference material is preferably selected to be at least equal to three times the optical attenuation length that the protective coating has for the measurement wavelength of the radiation detector.
In order to achieve a temperature measurement that is independent of emissivity, in accordance with a particularly preferred embodiment of the invention the reference substance is heated with at least one radiant heat source whose emitted radiation is modulated, advantageously actively modulated, with at least one characteristic parameter, whereby the radiation emitted from the at least one radiant heat source is detected with at least one second radiation detector, and whereby the radiation detected by the first radiation detector is corrected by the radiation detected from the second radiation detector to compensate the radiation of the radiation source that is reflected by the reference substrate.
Because of the characteristic known modulation of the radiation source, it is possible to differentiate between the radiation emitted by the article itself, which is necessary for determining the temperature of the reference substrate, and the reflected radiation of the radiation source. For additional advantages and details with respect to the modulation and evaluation of the radiation of the radiation source, known as the xe2x80x9cripplexe2x80x9d technique, refer to U.S. Pat. No. 5,490,728 and to DE 197 54 386 A (not previously published) from the same applicant, the contents of which are incorporated into the instant specification.
In this manner it is possible to achieve calibration of the absolute temperature independent of knowing the emissivity of the reference substrate. In order to adjust the measurement system to be independent with respect to emissivity, TC wafers can be used to precalibrate, since in this case the issue is primarily minor deviation between the TCs.
Advantageously, the modulation is used to characterize the radiation emitted from the radiation source when the radiation detected by the first radiation detector is being corrected, which makes it possible to differentiate very simply, reliably, and precisely the radiation from the radiation that is actually to be measured and that is emitted by the reference substrate itself.
The radiation emitted from the radiation source is preferably amplitude, frequency, and/or phase-modulated. Depending on the specific qualities and requirements, the type of modulation is selectable, whereby the type of modulation is selectable in particular with respect to the simplicity and reliability of the modulation method, but also with regard to evaluation and detection methods.
In accordance with a preferred embodiment of the invention, the radiant heat source comprises a plurality of lamps, whereby at least the radiation from one of the lamps is modulated, but advantageously the radiation of all of the lamps is modulated.
Advantageously the degree of modulation or the depth of modulation is controlled, whereby this degree of modulation or depth of modulation is known, and the detection and evaluation are simplified.
The temperature measurement independent of emissivity can advantageously also be achieved in that the measurement of the heat radiation emitted by the substrate is taken on a side of the substrate that constitutes at least a part of a cavity radiator.
In accordance with another preferred embodiment of the invention, a plurality of reference materials, each with a different melting point, are provided on the reference substrate, and the measurement plateaus are determined during the heating and/or cooling period, and they are each compared to one of the known melting points.
The instant invention can be used to great advantage for calibrating a temperature measurement in conjunction with an apparatus for thermal treatment of substrates in an oven in which the substrates are heated and cooled rapidly and with the most accurate possible pre-specified temperatures.
The aforestated object is also achieved in accordance with the invention using an apparatus for calibrating temperature measurements of the type cited in the foregoing that has the following: a reference material with known melting point that is applied to a reference substrate, a source of radiation for heating the reference substrate, whose emitted radiation can be modulated, preferably actively, by means of a modulation device with at least one characteristic parameter, and at least one second radiation detector for measuring the radiation emitted by the at least one source of radiation, and a device for correcting the radiation detected by the first radiation detector, for compensating the radiation of the source of radiation reflected by the reference substrate by means of the radiation detected by the second radiation detector.
The use of a reference substrate with a reference material applied thereto that has a known melting point in combination with the active modulation results in the aforementioned advantages of simple calibration of a temperature measurement independent of emissivity.
In accordance with one embodiment of the invention, the apparatus has a cavity radiator that is formed at least partially by the substrate.
The cavity radiator is preferably formed by a mirrored chamber whose one wall is formed at least in part by the substrate. Advantageously the cavity radiator can also be formed by a plate that is arranged parallel to the substrate.
In accordance with a preferred exemplary embodiment, provided on the reference substrate is a covering that provides a chamber for receiving the reference material between itself and the reference substrate. Preferably the chamber is sealed against its surroundings in order to prevent the reference material from contaminating the apparatus to be calibrated. In order to provide uniform pressure (partial pressure of the reference material) in the chamber, it is advantageously evacuated, which is also associated with the advantage that no liquid reference material is pressed out through any extant microtears under the pressure of heated residual gas. The entire reference substrate is advantageously glazed.
Advantageously the reference substrate has at least one depression for receiving the at least one reference material, whereby the bottom of the reference substrate in the region of the reference material is as thin as possible in order to keep as small as possible a temperature difference between the underside of the reference substrate and the reference material. A plurality of depressions is preferred for reasons of stability. The walls of the depressions are preferably inclined in order to avoid mechanical tensions due to different coefficients of expansion between the reference substrate and the reference material.
Advantageously the reference substrate has the same size and/or shape and/or the same weight as the substrate whose temperature is to be measured after the calibration in order to manage the reference substrate with an available handling system for the substrate to be measured. In this manner calibration or re-calibration can be automated. The advantage over conventional TC substrates is not only the increase in accuracy and reproducibility, but also that a TC substrate cannot be handled automatically due to the connection wires and the wires must be clamped on after the TC substrate has been inserted. This situation does not occur with the described reference substrate.
Advantageously the at least one reference material is a metal, since the melting points for metals, especially very pure metals, are known or defined very precisely.
In accordance with another embodiment of the invention, the different reference materials are provided with different melting points on the reference substrate in order to facilitate calibration at different temperatures.
Advantageously at least one reference material is arranged at different sites on the reference substrate, thus making it possible to ensure that the reference material covers the field of view of the first radiation detector.
Advantageously the reference substrate carries the at least one reference material on a side facing away from the radiation detector. In order to achieve high emissivity in the reference substrate, structures, such as microchannels, are advantageously provided on a side of the reference substrate that faces the radiation detector.
A plurality of reference substrates with different optical coatings are provided in order to obtain primary references with different emissivities. By providing primary references with different emissivities, it is possible to dispense completely with complex precalibration using TC wafers.
Due to the relatively simple manufacture and thermal properties of ceramic material, the reference wafer is preferably made thereof.
The instant invention is highly suitable for calibrating an apparatus for rapid thermal treatment of substrates in an oven in which the substrates are rapidly heated and cooled at the most precise possible pre-specified temperatures.